1. Field of the Invention
The present invention relates to power supplies with remotely located loads and more particularly to circuits adapted for use in those supplies which sense the voltage at the load as opposed to the output terminals of the supply.
2. Description of the Prior Art
In power systems the load may be located at a point which is not physically close to the output terminals of the supply. Such a non-physically close load will be referred to hereinafter by the term "remote". In every power system the load is external to the output terminals of the supply. Ordinarily the load will be within a few feet of the supply. As the operating frequency of the supply increases, the distance that the load has to be located from the supply output terminals in order to be referred to as remote decreases.
The leads connecting the load to the supply output terminals have both resistance and inductance which form, in combination with any capacitance in the load, a filter. In addition, it may also be desirable to sense the voltage at the load as opposed to sensing at the output terminals of the supply where sensing traditionally occurs. In such circumstances a pair of sensing leads either alone or as part of a wiring harness is extended to the load from the supply. These sensing leads form part of a control loop which is used to regulate the output voltage of the supply at the load. The filter described above, which arises from the combination of the resistance and inductance of the leads connecting the load to the supply output terminals and the capacitance of the load, introduces an undesirable phase shift in this control loop. This phase shift may be significant depending on how far the load is from the supply output terminals and the operating frequency of the supply. It is, therefore, desirable to minimize the effect that the undesired phase shift may have on the voltage control loop of a power supply and in particular those supplies that operate at a high frequency without compromising the desirability of sensing the voltage at the load. The term high frequency refers to those supplies which operate at a frequency which is above the audible range. Such supplies typically operate at a frequency of 20 kHz and above.
Additionally, the sense leads themselves, because of their length, may introduce erroneous signals in the form of noise into the voltage control loop. Terminating the ends of the sense leads at the supply with a capacitor may allow stray a-c signals picked up in the leads to be bypassed to ground but in turn introduces in the control loop in combination with the resistance and inductance of the sense leads additional undesirable phase shift. Use of sense leads which are both shielded and in the form of a twisted pair may minimize some of the effects that the additional undesirable phase shift introduces in the control loop. While the use of such sense leads may be effective, they do, however, add to the time for and cost of manufacturing and installing the supply. It is, therefore, also desirable that the effect of any additional phase shift and noise introduced by the sense leads themselves be minimized in a cost effective manner without compromising the desirability of sensing the voltage at the load.
Finally, as the sense leads extend some distance from the supply to the load, they then can be subject to breakage or accidental disconnection for any one of a number of reasons. Upon the breaking or accidental disconnection of a sense lead, the supply output voltage may either increase or decrease from the regulated value. While most supplies include well-known prior art circuits to sense a relatively large undervoltage or overvoltage condition in the supply's output voltage, it may, therefore, be additionally desirable to detect the breaking of the sense lead before the output voltage reaches the undervoltage or overvoltage sensing circuit threshold. Upon the detection of such a break, a suitable alarm condition may then be indicated.